This follows from
If you wish to see why 'P(A1 n A2) + P(A1 n A3) + P(A2 n A3) - P(A1 n A2 n A3) is positive'
draw a Venn diagram http://www.learnnc.org/reference/Venn%20diagram
The triple intersection is contained repeatedly in those other probabilities.
Note that if then .
This comes from .